1. Field of the Invention
The present invention relates to non-invasive measurement of internal information in a scattering medium by causing pulsed light, square wave light, or continuous light to be incident on a scattering medium such as a living body, and detecting light propagating in the scattering medium. More particularly, the present invention relates to a method for measuring internal information in a scattering medium and an apparatus for the same, which are capable of measuring a ratio of absorption coefficients and a ratio of concentrations of specific absorptive constituents in the scattering medium, its time change, its spatial distribution, etc., and which are capable of improving measurement precision.
2. Related Background Art
Demands for measurement of an absorption constituent in a scattering medium such as a living body or improvement of the measurement are very strong, and there are several reports and attempts therefor. The main prior art are listed at the end of this section.
In general, since light is scattered and/or absorbed at random in a scattering medium, light does not propagate straight. For the scattering medium in which the absorption is zero, a total quantity of light never decreases, but since light is scattered by scattering constituents at random, light propagates in a zigzag manner. In this case, a distance that light can propagate without the effect of the scattering is called a mean free path or a mean diffusion length, which is an inverse of a transport scattering coefficient .mu..sub.S '. In the case of the living body specimen, the mean free path is about 2 mm. This is taught by Wilson Olsby Optical Reflectance and Transmittance of Tissues: Principle and Application, IEEE, J. Quantum Electron., Vol. 26, No. 12, pp. 2186-2199 (1990). In addition, the scattering medium comprises absorptive constituents other than the scattering constituents, so that the absorption occurs in accordance with a distance that light propagates while scattering, and the quantity of light is exponentially attenuated in accordance with the distance.
As prior art in the field of precise measurement of an absorption coefficient or a transport scattering coefficient of a scattering medium, it is necessary to measure the quantity of transmitted light or reflected light corresponding to continuous incident light or pulsed incident light, and it is necessary to measure transmitted light or reflected light corresponding to the pulsed incident light and to analyze its waveform. The former utilizes a measurement of the absorbance in which the Lambert-Beer Law is a basic principle. Here, the Lamber-Beer law is that the absorbance of the specimen is proportional to the product of the molar absorption coefficient, molar concentration, and a specimen thickness, and that a difference between the absorbances is proportional to a difference between the concentrations where a thickness of the specimen is constant.
However, in the case of the absorbance measurement in the scattering medium, a mean optical pathlength of light diffused during propagation between a light incident position and a photodetection point is varied depending on the absorption coefficient .mu..sub.a of the scattering medium. Therefore, in the absorbance measurement corresponding to the scattering medium in which the optical pathlength is constant, absorption coefficient dependency of the mean optical pathlength is a big problem, which hinders the precise measurement of an absorption coefficient or the concentration of an absorptive constituent, or if the measurement is performed, due to large errors in measurement, it cannot practically be used. For example, according to "Japanese Patent Application No. Sho 62-248590", "Japanese Patent Application No. Sho 62-336197", and "Japanese Patent Application No. Hei 2-231378", since a basic principle is to measure an optical coefficient in which an optical pathlength is assumed to be constant, errors in measurement caused by a change of the above-described optical pathlength cannot be avoided.
There is another method utilizing a mean optical pathlength which is measured by another method in the case of an absorbance measurement, but hence a mean optical pathlength is varied depending on the absorption coefficient, errors caused by approximating the mean optical pathlength to be a constant value cannot be avoided. Further, there are a method of measuring an absorbance difference using pulsed light, a method further utilizing a principle of dual-wavelength spectroscopy, and a method of measuring an absorbance using lights having three or more kinds of wavelengths, but in either case, since a method of measuring an absorbance optical density in which an optical pathlength in a scattering medium is assumed to be constant is applied, errors caused by the change of the optical pathlength occur in all cases, which hinders sufficient precise measurement.
In the prior art not based on an absorbance measurement, a method, in which transmitted light or reflected light is measured by time-resolved measurement using pulsed light or modulated light to analyze waveforms, has disadvantages that owing to the time-resolved measurement, a measurement method and an apparatus are very complicated and that the apparatus is expensive, as compared with a measurement method and an apparatus of the present invention in which quantity of light, that is, time integration of an optical signal is measured. There are several attempts for measuring internal absorption information by measuring reflected light or transmitted light upon the incidence of pulsed light to the medium by the time-resolved measurement and analyzing its waveform. Such is disclosed in Patterson et al., Time Resolved Reflectance and Transmittance for the Non-Invasive Measurement of Tissue Optical Properties, Applied Optics, Vol. 28, No. 12, pp. 2331-2336 (1989); Patterson et al., Application of Time-Resolved Light Scattering Measurements to Photodynamic Therapy Dosimetry, Proc. SPIE, Vol. 1203, pp. 670-675 (1990); and Sevick et al., Time-Dependent Photon Migration Imaging, Proc. SPIE, Vol. 1599, pp. 273-283 (1991). A measured optical signal has a long decay tail by the influence of scattering and absorption constituents. Patterson et al. assumes a model of uniform scattering medium to analytically obtain the light signal output.sup.3) . A wave representing a time change in intensity of the optical output signal given by the formula defined by patterson et al. matches a waveform obtained by an experiment using a uniform scattering medium. According to Patterson et al. and the results of the experiment by the inventors of the present application, the absorption coefficient of absorptive constituents in the scattering medium is given by slope of waveform (differential coefficient) obtained when the optical signal is sufficiently attenuated, i.e., when a sufficiently long period has elapsed. However, because the optical signal at a location where an absorption coefficient is obtained required to be sufficiently attenuated means that the signal is very feeble, a signal to noise ratio (S/N ratio) of the signal to be measured is decreased, and consequently errors in measurement are increased. Therefore, it is hard to use in practical application. There are several kinds of attempts other than the above, but none of them give sufficient measurement precision.